Present day radio data communication systems typically include a centrally located base station or central controller coupled to a host computer, which base or central controlled communicates to a plurality of system remote data terminal units over an outbound communication channel. Conversely, the system remote data terminal units communicate to the central site over a separate inbound channel.
Usually such communication is effected from the remote data terminal side in essentially pseudo-automatic fashion. That is, the user/radio operator merely attempts to access the communication channel and transmit data by pushing a button on the terminal itself. If the channel is clear, the terminal unit commences to transmit upon activation of the push to talk button. If the channel is determined in use, the initiating radio terminal unit tries again some other time, usually on a random basis. Provision is made to, for the most part, avoid destructive collisions that would otherwise occur when more than one radio data terminal attempts to transmit on a communication channel simultaneously. This avoidance is effected mainly by the establishment and use of a suitable operating protocol which in essence determines the radio traffic rules for such system.
An early innovation in the radio data communication requires the central station or controller to insert "busy bits" in the traffic forming the outbound channel message stream and, in this manner, advise any of the radio data terminal units that in fact the inbound channel is in use as indicated by the mere presence of the "busy bits".
Accordingly, in such radio data communication systems, it is obviously an objective to quickly identify the presence of data as well as to effect symbol timing (clock) recovery in a minimum amount of time. These factors directly impact the "throughput" or observable efficiency of the radio data communication system. In point of fact, the time required to detect the mere presence of data on the inbound channel constitutes a major contributor to the collision window. It will be appreciated that it is critical, to say the least, that the modem at the base station be able to detect data presence and then set the "busy bits" in the outbound message screen in the least amount of time. This optimization of the collision window obviously permits higher channel throughput, in an essentially exponential relationship. In optimizing this collision window, there are at least two major considerations, namely: 1) fast detect of the presence of a specific baseband modulation as well as 2) the detection of the symbol timing center of the received waveforms.